This invention relates to the field of refrigeration units which requires the periodic removal of frost from the evaporator heat transfer surfaces and more specifically to modular refrigeration units which are cooled by a liquid medium.
The conventional practice for distributing refrigeration over a wide area has been to locate the compressors and condensers in a central area and then connect these components to evaporators which are located adjacent to the refrigeration requirement. The most common example of this condition is the supermarket which typical would locate the compressors and condensers in a machine room in the rear of the building, to be connected with refrigeration pipes to evaporators located in cold cabinets positioned on the sales floor. But this common practice requires a large amount of refrigerant to fill the connecting pipes and is prone to refrigerant leakage from the multitude of joints which connect the pipes. Since common refrigerants are now known to be harmful to the earth's atmosphere, causing ozone depletion and global warming, alternative refrigeration strategies are being applied which reduce the amount of refrigerant used by refrigeration systems. A highly effective strategy, in particular for supermarkets, is to locate all of the refrigeration components adjacent to the refrigeration requirement and then cool the condenser with a heat transfer fluid such as water. In this manner, the extensive network of refrigeration pipes is eliminated and the potential for refrigeration leakage is substantially reduced.
This close-coupled assembly of refrigeration components is called a refrigeration nit for the purpose of the present patent application, but is also referred to as a condensing unit within the refrigeration trade. The as-described cooling of distributed refrigeration units with a cooling fluid is well understood by refrigeration practitioners and fluid-cooled refrigeration units can be readily purchased from refrigeration equipment manufacturers. And a review of patent history indicated that several attributes and improvements have been applied to this standard-practice technique. For example, U.S. Pat. No. 4,280,335 to Perez and U.S. Pat. No. 5,335,508 disclose the implementation of ice storage in conjunction with fluid-cooled refrigeration units in an attempt to utilize inexpensive off-peak electricity. U.S. Pat. No. 4,732,007 to Dolan et al. describes the use of multiple cooling fluids applied to refrigeration units in order to facilitate the retrofitting of existing refrigeration installations and allow for greater operating flexibility. And U.S. Pat. No. 5,440,894 to Schaeffer et al, discloses the implementation of fluid-cooled refrigeration units positioned adjacent to supermarket display fixtures in order to minimize the requisite amount of refrigerant.
In summary, id-cooled refrigeration units offer an effective means for reducing the amount of refrigerant required for distributed refrigeration applications and are currently being installed for this purpose. Examples of these fluid cooled refrigeration units are the Hussman Protocol described by Hussman Bulletin 0107_370_protocolco and the Hill Phoenix InviroPac described by Hill Phoenix Bulletin RS-D01_HPIP Based on the well-understood laws of thermodynamics as explained by Fundamentals of Classical Thermodynamics by Van Wylen et al., these fluid-cooled refrigeration units strive to operate with the lowest possible cooling fluid temperature in order to achieve the lowest possible condensing temperature and subsequently the highest possible efficiency. So during periods of cold weather, the fluid is cooled by the ambient air to as low as 40 F in order to achieve a nominal 50 F condensing temperature, assuming a typically 10 F differential between the condensing temperature and the cooling fluid temperature. In likewise fashion, the fluid could be cooled by an auxiliary refrigeration system such as a chiller to as low as 40 F in order to achieve a nominal 50 F condensing temperature and thus minimize the power requirements for the distributed fluid-cooled refrigeration units.
In review of well-understood refrigeration practice, the typical evaporator collects frost during its normal operation and this frost must be removed on a periodic basis with the application of external heat. A simple and common method for applying this external heat is to embed electric resistance heaters into the evaporator but clearly this method is disadvantaged by use of a substantial amount of expensive electrical energy. This waste of electricity can be avoided by implementing gas defrost in lieu of electric defrost. Methods which perform evaporator defrosting using refrigerant gas are well established by open-source technical publications. As stated by ASHRAE Handbook-Refrigeration-2010, Chapter 15: Retail Food Store and Equipment, compressor discharge gas or gas from the top of the warm receiver at saturated conditions can be directed to the evaporators that require defrosting. And a review of technical literature and patent history indicates that many embellishments to the basic concept have been conceived. For example, during basic gas defrost, the gas can condense to a liquid state and subsequently cause damage to the compressor. To remedy this condition, U.S. Pat. No. 4,318,277 to Cann et al. describes an accumulator for capturing liquid refrigerant returning to the compressor and then the utilization of hot gas from the compressor to vaporize the captured liquid refrigerant. U.S. Pat. No. 3,838,723 to Kramer explains the application of a heater for re-evaporating the captured liquid. And in similar fashion, the Kramer Thermobank concept as described by Kramer Bulletin TT1-803 uses a water tank which is heated by compressor gas for re-evaporating the captured liquid. And most importantly, U.S. patent application Ser. No. 13/560,242 to Boyko discloses a highly effective gas defrost system which is the method of gas defrost preferred by the present inventor.
The present invention relates to a system of fluid-cooled refrigeration units which use gas defrost, ranging in scope from one refrigeration unit to many refrigeration units. In order to fully understand the disclosure of the present invention, the standard-practice system for cooling fluid-cooled refrigeration units is first reviewed.
FIG. 1 shows a common and well-understood system for cooling either a single or multiple fluid-cooled refrigeration units. Each refrigeration unit contains a condenser 13 which must reject heat away from the refrigeration unit during the refrigeration process and this heat is typically called the “heat-of-rejection”. Condenser 13 is a heat exchanger with a refrigerant-side and a fluid-side. The fluid inlet for condenser 13 for each refrigeration unit is connected to condenser fluid supply pipe 100 and the fluid outlet of condenser 13 for each refrigeration unit is connected to condenser fluid return pipe 101. Condenser fluid return pipe 101 is connected to the inlet of cooling unit 102. Cooling unit 102 is a fluid chiller, cooling tower or similar cooling device. The outlet of cooling unit 102 is connected to the inlet of condenser fluid pump 103. The outlet of condenser fluid pump 103 is connected to condenser fluid supply pipe 100. Each condenser 13 condenser fluid supply pipe 100, condenser fluid return pipe 101, cooling unit 102 and condenser fluid pump 103 are filled with condenser fluid 104, which is a common heat transfer liquid such as water or glycol. Then, when condenser fluid pump 103 is energized, condenser fluid 104 recirculates between condensers 13 to cooling unit 102 and thus transfers the heat-of-rejection away from condensers 13 to cooling unit 102.
A common feature of all gas defrost systems is the requirement that the condensing temperature must be substantially greater than 32 F, the melting point of frost. This elevated condensing temperature is necessary to adequately transfer heat to the evaporator and complete the defrost process within a short period of time. Based on a review of common refrigeration practice, it is generally perceived that the condensing temperature necessary for effective defrost should be in the range of 80 F. But the potential efficiency improvement achieved by a low condensing temperature is substantial, as shown by FIG. 2 which provides a graphical presentation of efficiency as a function of condensing temperature. FIG. 2 delineates efficiency in terms of Coefficient of Performance (COP) which is calculated as the dimensionless ratio of the refrigeration effect divided by the compressor power. The efficiency differential can be extracted from FIG. 2 which shows that the COP at 50 F condensing is 1.8 times greater than at 80 F condensing with +20 F evaporator temperature and the COP at 50 F condensing is 1.5 times greater than at 80 F condensing with −20 F evaporator temperature.
In summary, a review of technical literature and prior art shows that distribution of fluid-cooled refrigeration units provides a highly effective method for reducing the emission of refrigerant into the atmosphere and thereby should be actively pursued as a means for reducing atmospheric ozone depletion, global warming and, of course, the operational cost due to the lost refrigerant. Nevertheless, current practice does not provide a system for applying gas defrost to fluid-cooled refrigeration units which can provide both quick defrosting and high thermodynamic efficiency by virtue of a low temperature condensing fluid. Therefore, what is needed is a gas-defrost system applicable to fluid-cooled refrigeration units which is not detrimentally impacted by a low temperature condenser fluid. And in order to achieve commercial viability, what is further needed is a gas defrost system applicable to fluid-cooled refrigeration units which can be easily and reliably implemented.